What does it mean when someone leaves a penny on the table after eating in a restaurant?

It's a message to the server that their service was sub-par. It's considered a bigger slam than simply not leaving a tip at all.

it's an insult meaning the service sucked

It means they didn't forget to tip - but they thought the service was so bad that it only deserved the lowest possible tip.

It means they thought the service sucked and their showing they didn't forget to tip, but showing they hated the service.

Almost always left as a sign of poor service.

It means that the service was really bad. If no tip is left,then the waiter could think that such was just an oversight,but a penny....

It's a very sarcastic way to say the servide was bad. Better to have just left with no tip.

I believe it means extra good meal/service but needs to be added along w/ a tip and penny on top. I could be wrong, but that is what I have heard.

Since society's rules, generally, look to a 15-20% tip as appropriate for good service, I would think that leaving a penny on the table is a sign that the server was incompetent or that his/her service was much below expectations. OTOH, if an appropriate tip has been left using a credit card, the penny on the table might just mean that someone found it and thought it belonged to the server or that one misplaced the coin, or any other reason.